This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an improvement for ballasts that include a direct coupled output.
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
An electronic ballast typically includes a rectifier for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 kHz. Converting from alternating current to direct current is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. Some ballasts include a boost circuit between the rectifier and the filter capacitor for increasing the voltage to the lamp. Many electronic ballast use what is known as a xe2x80x9cflybackxe2x80x9d boost circuit in which the energy stored in an inductor is supplied to the filter capacitor as small pulses of current at high voltage, utilizing the xcex4i/xcex4t characteristic of an inductor to produce a high voltage. U.S. Pat. No. 3,265,930 (Powell) discloses such a ballast.
A modern electronic ballast typically includes an integrated circuit in the front end of the ballast to operate the boost circuit and provide power factor correction. xe2x80x9cPower factorxe2x80x9d is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e., indicating whether or not the voltage and current are sinusoidal and in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one). Many semiconductor devices not only provide suitable AC to DC conversion but provide a xe2x80x9cuniversalxe2x80x9d front capable of being connected directly to any line voltage between 120 and 277 volts. Typically, the low voltage (3-18 volts DC) needed for powering the integrated circuits within an electronic ballast is derived from a small auxiliary power supply coupled to the line input.
The inverter section in a typical electronic ballast includes what is known as a direct coupled output; that is, a pair of switching transistors connected in series between a high voltage rail and a low voltage rail or common rail. The transistors conduct alternately, producing a square wave at their junction that is converted into a sine wave by a series resonant circuit coupled to the junction. A load, e.g. one or more lamps in series, is coupled in parallel with the series resonant capacitor.
Typically at input voltages above 250 volts, and less noticeably at lower input voltages, an electronic ballast having a direct coupled output is subject to flickering from lamps while the ballast is in a quiescent state. That is, the ballast is coupled to a line input but the inverter section is turned off. With the inverter section shut off, the switching transistors are non-conducting, which should mean that the lamps are off, but the lamps flicker.
In view of the foregoing, it is therefore an object of the invention to prevent flicker while an electronic ballast having a direct coupled output is in a quiescent state.
The foregoing objects are achieved in this invention in which it has been discovered that the lamps are sporadically conducting current from the common rail to earthen ground through a fixture containing the lamps. A transistor is coupled in series in the current path to the common rail. When the ballast is placed in a quiescent state, the transistor is rendered non-conducting, thereby isolating the lamps from the common rail and preventing flicker. In accordance with another aspect of the invention, the control electrode of the transistor is coupled to a source of low voltage and the transistor is rendered non-conducting when the source of low voltage is turned off. The lamps can also be isolated from the common rail by using a semiconductor switch in the rectifier section or by referencing the output of the inverter to the high voltage rail.